We've had one for about 15 months now, and it was about baseball size (and about that color too!), now it's about dinner plate size!
Bigger tank may require higher lighting, so be prepared for that. Ours is in a 30 gal, we're almost done setting up for a 75, which a lot of the livestock in the the 30 will go into, but not the nem, it's staying right where it is with the clowns! Right now, only about 1/2 of our tank is usable for other things, like coral because the nem can reach almost 1/2 way across. It's beautiful!! Our sebae seems to really like the MH lighting and we only feed occassionally (once a week or so, sometimes less).
Have fun and good luck!